[AMDGPU][InferAddressSpaces] Only rewrite address-spaces that can be trivially...

[AMDGPU][InferAddressSpaces] Only rewrite address-spaces that can be trivially casted to flat for llvm.amdgcn.flat.atomic.{fadd,fmax,fmin}

The intrinsic @llvm.amdgcn.flat.atomic.{fadd,fmax,fmin} can only be
selected for flat address spaces (constant, flat and global).

This patch restricts the cases over which GCNTTIImpl::rewriteIntrinsicWithAddressSpace
rewrites the intrinsic.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D149938